1. Field of the Invention
The invention is generally related to methods of treatment planning using radiotherapy, and more particularly to methods of treatment planning with adequate biological weighting using hadron and/or proton beam radiotherapy.
2. Related Art
In recent years there has been a steady increase in clinical use of high energy hadron beams for patients afflicted with tumors and other medical conditions. It has been long known that protons differ from conventional radiation (photons, electrons) in their biological effectiveness. That is, to cause the same biological effect a lower dose of protons is required. Therefore, protons are more biologically effective. The relative biological effectiveness (RBE) is defined as the ratio of dose of reference radiation to the proton dose required to achieve the same biological effect.
In current clinical practice with therapeutic proton beams, a single RBE value (typically 1.1) is applied to all treatment plans, irrespective of depth of penetration, tissue, or any other particulars of the treatment. This is because previous studies indicated that RBE values were small in magnitude and because the variability of RBE with treatment parameters was believed to be within 10-20%. It was concluded that this variability was small relative to uncertainties associated with RBE values. New studies, however, reveal that the RBE variations can be as large as 100-300%, especially in the penumbra of the treatment volume delineation (“TVD”), e.g., the outline of the volume that conforms to the shape of the tumor.
Proton radiotherapy may use either mono-energetic proton beams or poly-energetic proton beams. Mono-energetic proton beams are characterized by a peak in their depth-dose distribution. This so-called Bragg peak is a result of an increasing energy deposition with the depth of penetration, leading to a maximum at the end of range of the proton beam. To obtain a good physical dose distribution for radiotherapy applications, the Bragg peak is spread out by passive or active beam modulation techniques to cover the target volume of a treatment site. Modulated beam profiles have a central flat region, which is used for treatment. A passive beam modulation technique utilizes scattering material placed upstream to change the beam energy. An active beam modulation technique changes the beam energy electronically.
Poly-energetic proton beams are characterized by variations in the amounts of energy delivered and by a three-dimensional localization of the dose. Moreover, non-uniform dose distributions from each poly-energetic proton beam may superimpose to give a desired dose in a target volume.
Beam modulation generates a broad spectrum of energies within the target volume, with the mean energy of protons decreasing with penetration distance. This results in a corresponding variation in linear energy transfer (LET), which increases with the depth of penetration. A biological dose deposited by the protons can be described as a product of proton fluence and LET. Thus, in proton beam radiotherapy a highly conformal high dose region is achieved by varying the proton fluence, and the energy spectrum, which is the essence of beam modulation. This highly conformal high dose region is the so-called spread-out Bragg peak (SOBP).
Determination of proton RBE at different points along the SOBP has been done in many centers, such as Cyclotron Research Center at Louvain-La Neuve and TRIUMF Cyclotron Research Center. Data has been reported on modulated proton beams with energies less than 100 MeV. These experiments used different approaches to assess the proton RBE, but all of them show that the RBE increases with depth within SOBP, with values ranging from 1.1 to 2.5.
Proton RBE depends on a number of factors including the type of tissue and the biological end point, the initial proton energy, the energy spread of the input proton beam, the depth of beam penetration, and the beam modulation technique. The RBE has been shown to increase with depth within the SOBP both theoretically and experimentally. This is partially attributed to the fact that the average proton energy decreases with depth within the SOBP. There are fewer investigations of the region beyond the distal point of the SOBP. These studies conclude that the RBE values continue to increase at the declining distal edge of the SOBP.
Recent studies irradiated human tumor SCC25 cells with a 65-MeV proton beam. Five positions along the beam line were simulated using Perspex plates of different thickness: one position, corresponding to the beam entrance, with 2 mm thick perspex, two along the SOBP at 15.6 and 25 mm, and two more measurements at the declining distal edge at 27.2 and 27.8 mm. Clonogenic survival of the irradiated cells and of their progeny was determined at various dose values at each position. Cobalt 60 (hereinafter, “60Co”) γ-rays were used as the reference radiation in this study.
RBE values obtained in this study increased with increasing depth. At the proximal part of SOBP, the RBE was evaluated to be close to 1.0. It reached the 1.2 value at the distal part of SOBP. Within the declining edge it continued to increase, and reached the value of about 1.4 at 27.2 mm, and 2 at 27.8 mm, where the relative dose was about 50% of that at the peak value. These RBE values were evaluated at the survival level given by 2 Gy γ-rays. For the progeny of irradiated cells, the RBE values were similar. The incidence of delayed effects increased with dose and with the depth within the beam. The results of this study show that at the distal declining edge of the beam, the RBE values increase significantly to an extent that is of practical significance when the region of treatment volume is close to sensitive tissues.
A second study was performed using the 62-MeV proton beam of the CATANA (Centro di Adro Terapia e Applicazzioni Nucleari Avanzati) facility. Cell survival of a resistant HTB 140 human melanoma cell line was studied using various biological assays, at several depths along the SOBP, and at the declining distal edge. The three different assays used in this study were the clonogenic assay, the microtetrasolium assay, and the sulforhodamine B assay. To simulate different positions along the beam line, Perspex plates of various thicknesses were interposed. Cell samples were irradiated at 6.6, 16.3, 25, and 26 mm depths. The distal end of SOBP was at the depth of 25 mm that had a corresponding 102±3% relative dose, while the relative dose along the declining distal edge at 26 mm was 32±4%.
Surviving fractions at 2 Gy (SF2) were obtained throughout the whole SOBP, which indicated high level of radio-resistance of these cells. The RBE at 2 Gy was used to analyze the efficiency of proton irradiation to inactivate cells as compared to conventional γ-ray radiation.
The results of this study again showed considerable increase in RBE values when approaching the distal end of SOBP. It was found that at the declining distal edge of SOBP, where the relative dose was ≈32%, the killing ability of protons was close to that observed at the distal end of SOBP, where the relative dose was ≈102%. The RBE at this depth on declining distal edge was found to be close to 4, using the clonogenic assay, and close to 3, using the sulforhodamine B assay. For reference, the RBE at the proximal part of SOBP was found to be close to 1.3, using both clonogenic and sulforhodamine B assays.
Results of the two studies discussed above evidence the importance of additional investigations of RBE along the SOBP, and in particular, at its declining distal edge. These findings also establish the necessity of development of treatment planning methods, which will incorporate adequate proton RBE's.